The disclosure relates generally to power generation systems, and more particularly, to systems and methods for cooling the exhaust gas of power generation systems.
Utility power producers use combined cycle (CC) power generation systems because of their inherent high efficiencies and installed cost advantage. CC power generation systems typically include a gas turbine, a heat recovery steam generator (HRSG), and a steam turbine. The heat recovery steam generator uses the hot exhaust gas from the gas turbine to create steam, which drives the steam turbine. The combination of a gas turbine and a steam turbine achieves greater efficiency than would be possible independently.
Operational flexibility to meet varying power grid demands at different times of the day is an important consideration in CC power generation systems. The issue becomes more important as intermittent energy sources such as solar and wind are integrated into the power grid. To this extent, CC power generation systems powered by fossil fuels must be capable of increasing/decreasing power output as required to accommodate such intermittent energy sources.
Non-steady state emissions from a CC power generation system (e.g., during start-up) are generally closely scrutinized by regulatory authorities. During start-up, emission control devices employing selective catalytic reduction (SCR) and carbon monoxide (CO) catalysts are not active. To avoid thermal stresses in the steam turbine, the gas turbine has to be held at a lower load to control the HRSG inlet temperature to around 700° F. Since emissions are higher at lower gas turbine loads and the emission control devices are not yet active, emissions during start-up can be an order of magnitude higher than those at steady state operation. Further, operating gas turbines at lower loads for a considerable amount of time reduces the power provided to the power grid during the crucial start-up period.
The efficiency and power output of gas turbines vary according to ambient conditions. The amount of these variations greatly affects electricity production, fuel consumption, and plant revenue. In general, the power output and efficiency of a gas turbine decrease as the ambient air temperature increases.
The power output of a gas turbine is directly proportional to and limited by the mass flow rate of compressed air provided by the compressor to the combustor of the gas turbine. The power output of a gas turbine decreases due to a reduction of the air mass flow rate delivered by the compressor to the combustor (the density of the air decreases as the ambient air temperature increases). Further, the efficiency of a gas turbine decreases because the compressor requires more power to compress air of higher temperature.